This invention relates to a communication control apparatus and, more particularly, to a communication control apparatus for use in receiving asynchronous transfer mode (ATM) cells in an ATM network system with a local area network (LAN) emulation function.
In the manner which will later be described in conjunction with FIG. 1, an ATM network system of the type described has a LAN emulation function, namely, a function which artificially carries out legacy LAN communication on an ATM network. In other words, the LAN emulation is also called MAC over ATM and emulates a legacy LAN on an ATM. The LAN may be an Ethernet, a Token Ring, or the like. That is to say, the LAN emulation is service to provide mutual connection between the legacy LAN and the ATM at a bridge level so that a legacy LAN terminal can use an ATM-LAN part just like a legacy segment. The LAN emulation has been standardized by an ATM forum.
The ATM network system with the LAM emulation function comprises an ATM network with the LAN emulation function, a plurality of legacy LAN terminals connected to the ATM network, a plurality of ATM terminals, and a plurality of ATM bridges for connecting the legacy LAN terminals with the ATM network. In this event, each ATM terminal is generally provided with an ATM communication control apparatus for transmitting and receiving ATM cells.
In the manner known in the art, it is to be noted that each of the ATM cells is composed of fifty-three bytes and has a cell header field of five bytes and a cell payload field of forty-eight bytes. The cell payload field includes user data or a payload. The cell header field is assigned with a header, such as a virtual path identifier (VPI) or a virtual channel identifier (VCI) for identifying a virtual path or a virtual channel to be delivered. That is, a combination of VPI/VCI serves as an identifier (the number) for identifying a virtual connection that is embedded in the ATM header field. By using the VPI/VCI in the ATM header field, it is possible to realize communication on the ATM cells between transmission terminals and reception terminals.
In addition, transmission ATM cells are converted by the ATM bridges into a reception LAN packet which is sent to the legacy LAN terminal. On the other hand, each of the LAN terminals transmits a transmission LAN packet to an ATM bridge which converts the transmission LAN packet into reception ATM cells received in the ATM terminal. Each of the LAN packets has a packet format which is defined, for example, by IEEE802.3/Ethernet or by IEEE802.5/Token-Ring. Accordingly, the ATM terminals receives the reception ATM cells having the cell header fields and the cell payload fields where the transmission LAN packet is segmented every forty-eight bytes. Each of the LAN packets has a packet header field for storing a LAN emulation client (LEC) ID of two bytes and a medium access control (MAC) address of six bytes. Accordingly, each of the LAN packets is segmented into the segmented parts whose first one is included in the cell payload field of a first ATM cell. In other words, the first ATM cell has the cell header field and the cell payload field for storing the LEC ID/MAC address.
On the ATM network with the LAN emulation function, there is various transmission LAN packets transmitted in accordance with unicast service, broadcast service, or multicast service with the transmission LAN packets encapsulated into reception ATM cells. In other words, the ATM terminal or an ATM communication control apparatus receives the various transmission LAN packets as the reception ATM cells or terminal reception packets. The transmission LAN packets or the terminal reception packets are classified into two groups which will herein be called a first and a second group. The first group of the transmission LAN packets is unnecessary ones to be received in the ATM terminal. The second group of the transmission LAN packets is necessary ones to be received in the ATM terminal. For example, the first group of the transmission LAN packets includes transmission LAN packets which the ATM terminal itself transmits or transmission LAN packets each having the LEC ID assigned to each channel of the ATM terminal. In addition, the first group of the transmission LAN packets further includes transmission LAN packets each having the MAC address for other destinations except the ATM terminal.
In the manner which will later be described in conjunction with FIG. 4, a conventional ATM terminal or a conventional ATM communication control apparatus comprises a physical layer device, a segmentation and reassembly (SAR) receiving section, a SAR transmitting section, a direct memory access (DMA) controller, a system bus, a central processing unit (CPU), and a system memory. Connected to the ATM network, the physical layer device has a function of an ATM physical layer. Connected to the physical layer device, the SAR receiving section receives the reception ATM cells supplied from the ATM network via the physical layer device. The SAR receiving section includes a reception first-in first-out (FIFO) memory for selectively storing the reception ATM cells as stored cells in the manner which will later be described. On the basis of a value of the identifier in the header field of each of the reception ATM cells, the SAR receiving section carries out decision of reception, verification for various errors, and reassembly of the terminal reception packets.
Connected to the physical layer device, the SAR transmitting section carries out cell segmentation of terminal transmission packets to be transmitted, rate control for the transmission ATM cells, and so on. The SAR transmitting section transmits the transmission ATM cells to the ATM network via the physical layer device. Connected to the SAR receiving section, to the SAR transmitting section, and to the system bus, the DMA controller carries out interface control between the SAR receiving section and the system bus and between the SAR transmitting section and the system bus. The CPU and the system memory are connected to the system bus.
Description will proceed to reception operation in the conventional ATM terminal. It will be assumed that the SAR receiving section in the ATM terminal is supplied from the ATM network via the physical layer device with the reception ATM cells. In this event, the SAR receiving section determines whether the cell payload field in each reception ATM cell should be received or discarded by identifying only a value of the VPI/VCI in the cell header field of the reception ATM cell without identifying contents of the cell payload field in which the LEC ID/MAC address of the transmission LAN packet is written.
When the SAR receiving section determines that the cell payload field of the reception ATM cell should be received on the basis of the value of the VPI/VCI, the cell payload field of the reception ATM cell where reception is allowed is written in the reception FIFO memory as a stored payload field. Thereafter, the DMA controller reads the stored payload field out of the reception FIFO memory as a read payload field. The DMA controller transfers the read payload field to the system memory via the system bus to write the read payload field in the system memory as a written payload field. The CPU identifies the LEC ID/MAC address of the written payload field in the system memory to finally determine whether the written payload field should be received or discarded.
As described above, in the conventional communication control apparatus, the CPU identifies the LEC ID/MAC address in the payload written in the system memory to carry out discard of the unnecessary reception packets. In addition, load takes to the system bus due to DMA transfer of the unnecessary reception packets. As a result, the conventional communication control apparatus is disadvantageous in that it degrades performance for reception processing.
In addition, the conventional communication control apparatus is disadvantageous in that it wastes vacant areas in the reception FIFO memory and in the system memory when it receives the ATM cells unnecessary to receive. More specifically, in a case where the reception FIFO memory is put into a full state by storing the ATM cells unnecessary to receive, the reception FIFO memory cannot store effective ATM cells which must receive rightfully.
Thus in summary, the SAR receiving section of the conventional communication apparatus carries out decision of reception by identifying only the value of the VPI/VCI in the cell header field of the reception ATM cell without identifying the LEC ID/MAC address for the transmission LAN packet that is included in the cell payload of the first ATM cell for the transmission LAN packet.
It is therefore an object of the present invention to provide a communication control apparatus which is capable of preventing degradation of reception processing in a central processing unit although it receives an LAN packet which is unnecessary to receive.
Other objects of this invention will become clear as the description proceeds.
According to an aspect of this invention, a method is for receiving a cell which has a header field for storing an identifier and a payload field for storing a payload. The method comprises the step of determining whether the cell should be received or be discarded on the basis not only of the identifier in the header field but also of a specific address in the payload included in the payload field to produce a determined result indicative of one of reception and discard for the cell, of storing the cell in a memory when the determined result indicates the reception for the cell, and of discarding the cell without storing it in the memory when the determined result indicates the discard for the cell.
According to another aspect of this invention, a method is for receiving cells into which a packet is segmented. Each of the cells has a header field for storing an identifier and a payload field for storing a payload. One of the cells is a first cell for the packet. The method comprises the step of determining whether the packet should be received or be discarded on the basis not only of the identifier in the header field but also of a specific address in the payload included in the payload field of the first cell to produce a determined result indicative of one of reception and discard for the packet, of storing the packet in a memory when the determined result indicates the reception for the packet, and of discarding the packet without storing it in the memory when the determined result indicates the discard for the packet.
According to still another aspect of this invention, a communication control apparatus receives reception cells and transmits transmission cells. Each of the reception cells and the transmission cells has a header field assigned with an identifier and a payload field assigned with a payload. The communication control apparatus comprises a physical layer device connected to a network. The physical layer device has a function of a physical layer in an asynchronous transfer mode (ATM). Connected to the physical layer device, a receiving section receives the reception cells supplied from the network via the physical layer device. The receiving section includes a memory for selectively storing the reception cells as stored cells. On the basis of a value of the identifier in the header field of each of the reception cells, the receiving section carries out decision of reception, verification for various errors, and reassembly of a reception packet. Connected to the physical layer device, a transmitting section carries out cell segmentation of a transmission packet to be transmitted and rate control for the transmission cells. The transmitting section transmits the transmission cells to the network via the physical layer device. Connected to the receiving section, the transmitting section, and a system bus, a direct memory access (DMA) controller interfaces the receiving section and the transmitting section with the system bus. The receiving section further comprises address filtering means connected to the physical layer device. The address filtering means identifies a value of a particular address in the payload included in the payload field of a first cell for the reception packet to produce an address filtered signal indicative of an identified value. Connected to the address filtering means and the memory, write-in control means determines whether or not the reception packet should be received on the basis of the identified value indicated by the address filtered signal. The write-in control means writes the reception packet in the memory when the reception packet should be received. The write-in control means discards the reception packet without writing it in the memory when the reception packet should be not received.
According to yet another aspect of this invention, a receiving unit receives reception cells each of which has a header field assigned with an identifier and a payload field assigned with a payload. The receiving unit includes a memory for selectively storing the reception cells as stored cells. On the basis of a value of the identifier in the header field of each of the reception cells, the receiving unit carries out decision of reception, verification for various errors, and reassembly of a reception packet. The receiving unit further comprises address filtering means for identifying a value of a particular address in the payload included in the payload field of a first cell for the reception packet to produce an address filtered signal indicative of an identified value. Connected to the address filtering means and the memory, write-in control means determines whether or not the reception packet should be received on the basis of the identified value indicated by the address filtered signal. The write-in control means writes the reception packet in the memory when the reception packet should be received. The write-in control means discards the reception packet without writing it in the memory when the reception packet should be not received.